Encoding Values by Pseudo-Random Mask

ABSTRACT

A method for a keyed cryptographic operation by a cryptographic system mapping an input message to an output message, including: receiving input data for the keyed cryptographic operation; calculating a first mask value based upon the input data; and applying the first mask value to a first intermediate value of the keyed cryptographic operation.

TECHNICAL FIELD

Various exemplary embodiments disclosed herein relate generally to securing software components that perform a cryptographic function against attacks including encoding values by pseudo-random masks.

BACKGROUND

The Internet provides users with convenient and ubiquitous access to digital content. Because the Internet is a powerful distribution channel, many user devices strive to directly access the Internet. The user devices may include a personal computer, laptop computer, set-top box, internet enabled media player, mobile telephone, smart phone, tablet, mobile hotspot, or any other device that is capable of accessing the Internet. The use of the Internet as a distribution medium for copyrighted content creates the compelling challenge to secure the interests of the content provider. Increasingly, user devices operate using a processor loaded with suitable software to render (playback) digital content, such as audio and/or video. Control of the playback software is one way to enforce the interests of the content owner including the terms and conditions under which the content may be used. Previously many user devices were closed systems. Today more and more platforms are partially open. Some users may be assumed to have complete control over and access to the hardware and software that provides access to the content and a large amount of time and resources to attack and bypass any content protection mechanisms. As a consequence, content providers must deliver content to legitimate users across a hostile network to a community where not all users or user devices can be trusted.

Secure software applications may be called upon to carry out various functions such as, for example, cryptographic functions used to protect and authenticate digital content. In order to counter attacks, these algorithms have to be obfuscated (hidden) in order to prevent reverse engineering and modification of the algorithm or prohibit obtaining the user-specific secure information. Accordingly, the functions of the secure software application may be carried out by various functions as defined by the instruction set of the processor implementing the secure software. For example, one way to obscure these functions is by the use of lookup tables.

Content providers must deliver content to legitimate users across a hostile network to a community where not all users or devices can be trusted. This has led to the development of white-box cryptography. In the white-box cryptography scenario it is assumed that the user has complete control of the hardware and software that provides access to the content, and an unlimited amount of time and resources to attack and bypass any content protection mechanisms. The secure software code that enforces the terms and conditions under which the content may be used should be tamper resistant. Digital rights management is a common application of secure software applications. The general approach in digital rights management for protected content distributed to user devices is to encrypt the digital content using for example, DES (Data Encryption Standard), AES (Advanced Encryption Standard), or using other known encryption schemes, and to use decryption keys to recover the digital content. These decryption keys must be protected to prevent unauthorized access to protected material.

In the digital right management scenario, the attacker has complete control of the software enforcing the management and access to the protected content. Accordingly, the attacker can modify software and also seek to obtain cryptographic keys used to encrypt the protected content. Such keys may be found by analyzing the software

Regarding key distribution, a media player has to retrieve a decryption key from a license database in order to play back the media. The media player then has to store this decryption key somewhere in memory for the decryption of the encrypted content. This leaves an attacker two options for an attack on the key. First, an attacker may reverse engineer the license database access function allowing the attacker to retrieve asset keys from all license databases. In this situation the attacker does not need to understand the internal working of the cryptographic function. Second, the attacker may observe accesses of the memory during content decryption, thus the attacker may retrieve the decryption key. In both cases the key is considered to be compromised.

The widespread use of digital rights management (DRM) and other secure software has given rise to the need for secure, tamper-resistant software that seeks to complicate tampering with the software. Various techniques for increasing the tamper resistance of software applications exist. Most of these techniques are based on hiding the embedded knowledge of the application by adding a veil of randomness and complexity in both the control and the data path of the software application. The idea behind this is that it becomes more difficult to extract information merely by code inspection. It is therefore more difficult to find the code that, for example, handles access and permission control of the secure application, and consequently to change it.

As used herein, white-box cryptography includes a secure software application that performs cryptographic functions in an environment where an attacker has complete control of the system running the white-box cryptography software. Thus, the attacker can modify inputs and outputs, track the operations of the software, sample and monitor memory used by the software at any time, and even modify the software. Accordingly, the secure functions need to be carried out in a manner that prevents the disclosure of secret information used in the secure functionality. White-box cryptography functions may be implemented in various ways. Such methods include: obscuring the software code; using complex mathematical functions that obscure the use of the secret information; using look-up tables; using finite state machines; or any other methods that carry out cryptographic functions but hide the secret information needed for those secure functions. A white-box implementation may also contain components that include anti-debugging and tamper-proofing properties.

There are several reasons for preferring a software implementation of a cryptographic algorithm to a hardware implementation. This may, for instance, be the case because a software solution is renewable if the keys leak out, because it is has lower cost, or because the application-developer has no influence on the hardware where the white-box system is implemented.

SUMMARY

A brief summary of various exemplary embodiments is presented below. Some simplifications and omissions may be made in the following summary, which is intended to highlight and introduce some aspects of the various exemplary embodiments, but not to limit the scope of the invention. Detailed descriptions of an exemplary embodiment adequate to allow those of ordinary skill in the art to make and use the inventive concepts will follow in later sections.

Various embodiments relate to a non-transitory machine-readable storage medium encoded with instructions for execution by a keyed cryptographic operation by a cryptographic system mapping an input message to an output message, including: instructions for receiving input data for the keyed cryptographic operation; instructions for calculating a first mask value based upon the input data; and instructions for applying the first mask value to a first intermediate value of the keyed cryptographic operation.

Various embodiments are described, further comprising setting a threshold value ε, wherein the mask value based upon the input is calculated such that for all values of b: |p_(a|b)−p_(a)|≦ε, where p_(a|b) is the number of input values for which the masked value equals a given that the first intermediate value equals b divided by the number of values for which the intermediate value equals b, and where p_(a) is the number of input values for which the masked value equals a divided by the number of input values.

Various embodiments are described, wherein the threshold value E is selected based upon an expected number of traces collected by an attacker.

Various embodiments are described, wherein the cryptographic function is the Advanced Encryption Standard (AES).

Various embodiments are described, wherein the cryptographic function is the Data Encryption Standard (DES).

Various embodiments are described, wherein the input data and has N portions and wherein instructions for calculating a first mask value based upon the input data further includes: instructions for applying N bijective functions to each of the N portions of the input data; and instructions for combining outputs of the N bijective functions resulting in the first mask value.

Various embodiments are described, further including: instructions for calculating second mask value based upon the input data that is different from the first mask value; and instructions for applying the second mask value to a second intermediate value of the keyed cryptographic operation.

Various embodiments are described wherein the keyed cryptographic operation includes a plurality of rounds with each round including a plurality of substitution functions, the outputs of the substitution functions are combined to produce an output of the round, and the outputs of the substitution functions are the first intermediate value.

Various embodiments are described, further including: applying the first mask value to the output of the round to remove the first mask value.

Various embodiments are described wherein the keyed cryptographic operation further includes: a first round of the keyed cryptographic operation producing a first masked output; and a second round receiving the first masked output wherein the second round compensates for the masking of the first masked output.

Further various embodiments relate to a method of controlling a server that provides an application that implements a keyed cryptographic operation by mapping an input message to an output message, including: receiving a request from a user for the application that implements a keyed cryptographic operation by mapping an input message to an output message; and providing the user the application that implements a keyed cryptographic operation by mapping an input message to an output message, wherein the application was created by: receiving input data for the keyed cryptographic operation; calculating a first mask value based upon the input data; and applying the first mask value to a first intermediate value of the keyed cryptographic operation.

Various embodiments are described wherein the application was further created by setting a threshold value ε, wherein the mask value based upon the input is calculated such that for all values of b: |p_(a|b)−p_(a)|≦ε, where p_(a|b) is the number of input values for which the masked value equals a given that the first intermediate value equals b divided by the number of values for which the intermediate value equals b, and where p_(a) is the number of input values for which the masked value equals a divided by the number of input values.

Various embodiments are described wherein the threshold value E is selected based upon an expected number of traces collected by an attacker.

Various embodiments are described wherein the input data and has N portions and wherein the application was further created by: applying N bijective functions to each of the N portions of the input data; and combining outputs of the N bijective functions resulting in the first mask value.

Various embodiments are described wherein the application was further created by: calculating second mask value based upon the input data that is different from the first mask value; and applying the second mask value to a second intermediate value of the keyed cryptographic operation.

Various embodiments are described wherein the keyed cryptographic operation includes a plurality of rounds with each round including a plurality of substitution functions, the outputs of the substitution functions are combined to produce an output of the round, and the outputs of the substitution functions are the first intermediate value.

Various embodiments are described wherein the application was further created by: applying the first mask value to the output of the round to remove the first mask value.

Various embodiments are described wherein the keyed cryptographic operation further includes: a first round of the keyed cryptographic operation producing a first masked output; and a second round receiving the first masked output wherein the second round compensates for the masking of the first masked output.

Further various embodiments relate to a method for a keyed cryptographic operation by a cryptographic system mapping an input message to an output message, including: receiving input data for the keyed cryptographic operation; calculating a first mask value based upon the input data; and applying the first mask value to a first intermediate value of the keyed cryptographic operation.

Various embodiments are described, further comprising setting a threshold value ε, wherein the mask value based upon the input is calculated such that for all values of b: |p_(a|b)−p_(a)|ε, where p_(a|b) is the number of input values for which the masked value equals a given that the first intermediate value equals b divided by the number of values for which the intermediate value equals b, and where p_(a) is the number of input values for which the masked value equals a divided by the number of input values.

Various embodiments are described wherein the threshold value E is selected based upon an expected number of traces collected by an attacker.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to better understand various exemplary embodiments, reference is made to the accompanying drawings, wherein:

FIG. 1 illustrates the main steps of a round of AES;

FIG. 2 illustrates a white-box AES implementation with fixed encodings on the input of the rounds;

FIG. 3 illustrates the computation of one output nibble by means of a network of look-up tables;

FIG. 4 illustrates a portion of the network table of FIG. 3 obfuscated by encoding the inputs and outputs;

FIG. 5 illustrates a illustrates a network that computes a pseudo-random mask m;

FIG. 6 illustrates how the mask may be applied to the intermediate values present in the network of tables in FIG. 3; and

FIG. 7 illustrates a system for providing a user device secure content and a software application that processes the secure content.

To facilitate understanding, identical reference numerals have been used to designate elements having substantially the same or similar structure and/or substantially the same or similar function.

DETAILED DESCRIPTION

The description and drawings illustrate the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within its scope. Furthermore, all examples recited herein are principally intended expressly to be for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Additionally, the term, “or,” as used herein, refers to a non-exclusive or (i.e., and/or), unless otherwise indicated (e.g., “or else” or “or in the alternative”). Also, the various embodiments described herein are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

There are several reasons for preferring a software implementation of a cryptographic algorithm to a hardware implementation. This may, for instance, be the case because a software solution is renewable if the keys leak out, because it is has lower cost, or because the application-developer has no influence on the hardware where the white-box system is implemented. While the description of embodiments below are directed to software implementation running on a processor, it is noted that these embodiments may also be partially or completely implemented in hardware as well. The lookup tables and finite state machines that are described may be implemented in hardware to carry out the various functions described.

A table-based approach to a white-box implementation of the Advanced Encryption Standard (AES) and the Data Encryption Standard (DES) were proposed in the following papers: “White-Box Cryptography and an AES Implementation”, by Stanley Chow, Philip Eisen, Harold Johnson, and Paul C. Van Oorschot, in Selected Areas in Cryptography: 9th Annual International Workshop, SAC 2002, St. John's, Newfoundland, Canada, Aug. 15-16, 2002, referred to hereinafter as “Chow 1”; and “A White-Box DES Implementation for DRM Applications”, by Stanley Chow, Phil Eisen, Harold Johnson, and Paul C. van Oorschot, in Digital Rights Management: ACM CCS-9 Workshop, DRM 2002, Washington, D.C., USA, Nov. 18, 2002, referred to hereinafter as “Chow 2”. Chow 1 and Chow 2 disclose methods of using a table-based approach to hide the cryptographic key by a combination of encoding its tables with random bijections, and extending the cryptographic boundary by pushing it out further into the containing application.

As noted, for many cryptographic operations it is desired to have a white-box implementation. The invention may be applied, for example, to symmetric and asymmetric cryptographic operations. Also, the invention may be applied to block ciphers, stream ciphers, message authentication schemes, signature schemes, etc. Note that the invention may also be applied to hash functions. The latter is especially useful if the hash function is used as a building block which processes secret information, e.g., a secret key, secret data, etc. For example, the invention may be applied to a hash function used in a keyed-Hash Message Authentication Code (HMAC or KHMAC). Well known block ciphers include: Advanced Encryption Standard (AES), Secure And Fast Encryption Routine, (SAFER, and variants SAFER+ and SAFER++), Blowfish, Data Encryption Standard (DES), etc. A well-known stream cipher is RC4. Moreover any block cipher can be used as stream cipher using an appropriate mode of operation, e.g., Cipher feedback (CFB), Counter mode (CTR), etc.

The input message can represent, e.g., encrypted content data, such as multi-media data, including audio and/or video data. The encrypted content data may also include encrypted software, e.g., encrypted computer code representing some computer application, e.g., a computer game, or an office application. The input message may also represent a key for use in a further cryptographic operation. The latter may be used, for example, in a key exchange protocol, wherein a white-box implementation according to the invention encrypts and/or decrypts data representing a new key. The input data may also be plain data, for example, plain user data. The latter is especially advantageous in message authentication schemes. A white-box implementation according to the invention may have the property that the implementation may only be used for encryption, only be used for decryption, but not for both. For example, this property can be achieved if the implementation uses look-up tables which are not bijective, for example, a look-up table having more input bits than output bits. Accordingly, if a user only has a white-box decryptor, he may verify a MAC code but not create new MACs. This strengthens the non-repudiation properties of such a message authentication scheme.

The white-box implementation may be implemented using a plurality of basic blocks. The plurality of basic blocks is interconnected, in the sense that some of the blocks build on the outputs of one or more of the previous blocks. A basic block may be implemented in hardware, for example, as a computer chip. A basic block may use a switch board, a state machine or any other suitable construction for implementing functions in computer hardware. A basic block may also be implemented in software running on a general purpose computer chip, e.g. a microprocessor. For example, a basic block may use a plurality of computer instructions, including arithmetical instructions, which together implement the functionality of the basic block. A widely used implementation for the basic block, which may be used both in software and hardware, is a look-up table. For example, Chow 1 and Chow 2 take this approach to implement the AES and DES block ciphers. A look-up table implementation includes a list which lists for possible input values, an output value. The input value may be explicit in the lookup table. In that situation the look-up table implementation could map a particular input to a particular output by searching in the list of input values for the particular input. When the particular input is found the particular output is then also found. For example, the particular output may be stored alongside the particular input. Preferably, the input values are not stored explicitly, but only implicitly. For example, if the possible inputs are a consecutive range, e.g. of numbers or bit-strings, the look-up table may be restricted to storing a list of the output values. A particular input number may, e.g., be mapped to the particular output which is stored at a location indicated by the number. Further, finite state machines or code obfuscation may be used to implement the white-box implementation.

For example, a look up table for a function may be created by computing the output value of the function for its possible inputs and storing the outputs in a list. If the function depends on multiple inputs the outputs may be computed and stored for all possible combinations of the multiple inputs. Look-up tables are especially suited to implement non-linear functions, which map inputs to output in irregular ways. A white-box implementation can be further obfuscated, as is explained below, by applying to one or more of its look-up tables a fixed obfuscating input encoding and a fixed output encodings. The results of applying a fixed obfuscating input encoding and output encodings is then fully pre-evaluated. Using this technique, a look-up table would be replaced by an obfuscated look-up table which has the same dimensions, that it takes the same number input bits and produces the same number of output bits. The input encoding and output encoding used in such obfuscation are not explicit in the final white-box implementation.

The network of basic blocks are arranged to compute an output message when they are presented with an input message. Typically, the input message is operated upon by a number of basic input blocks. A number of further basic blocks may take input from one or more of the basic input blocks and/or from the input. Yet further basic blocks can take input in any combination of the input message, the output of basic input blocks and the output of the further basic blocks. Finally some set of basic exit blocks, i.e., at least one, produce as output all or part of the output-message. In this manner a network of basic blocks emerges which collectively computes the mapping from the input message to output message.

The key used may be a cryptographic key and may contain sufficient entropy to withstand an anticipated brute force attack. It is noted that in a white-box implementation, the key is typically not explicitly present in the implementation. This would risk the key being found by inspection of the implementation. Typically, the key is only present implicitly. Various ways are known to hide a key in a cryptographic system. Typically, at least the method of partial evaluation is used, wherein a basic block which needs key input is evaluated in-so-far that it does not depend on the input-message. For example, a basic operation wherein an input-value, a masking value, which does not depend on the input-message, e.g. a value from a substitution box (S-box), and a key-value need to be XORed can be partially evaluated by XORing the key value and the masking value together beforehand. In this way the operation still depends on the key-value although the key-value is not explicitly present in the implementation. Instead, only the XOR between the key-value and masking-value is present in the implementation. Note that, more complicated ways and/or further ways of hiding the keys are compatible with embodiments of this invention.

An interesting category of attacks on white-box implementations is correlation power analysis (CPA). These type of attacks work as follows.

An attacker collects a large number of execution traces for the white-box implementation, each with a different, but known plaintext. In CPA attacks, this trace is a power trace, but the traces may measure other different properties of the white-box implementation. The traces may be placed in a matrix t where t_(i,j) denotes the trace-value (e.g., power consumption) at time point j for trace i.

Then, the attacker chooses some intermediate value v or a function ƒ(v) thereof in a standard implementation that has some relation to a limited number of key-bytes. Because ƒ(v) equals v if ƒ is the identity function, in the description below ƒ(v) is used for both cases.

The attacker then determines the value ƒ(v) in the standard implementation for different guesses/hypotheses of the key bits on which the value depends and for the different plaintexts for which execution trace data has been collected. For the ith plaintext and the jth key hypothesis, this gives the value ƒ(v_(i,j)).

For each key hypothesis k and time point p, the attacker determines the correlation between ƒ(v_(i,k)) and t_(i,p) over all traces. For the correct key-hypothesis, the correlation will typically be higher than for an incorrect key-hypothesis. If this is the case, then there is key leakage.

In hardware implementations where a white-box attack model is not assumed, attacks like this are typically prevented by adding random noise to the execution. This approach, however, does not work for white-box implementations because in a white-box attack model an adversary can disable the source generating the random data.

To harden an implementation against CPA-attacks, embodiments are described that mask intermediate values of a cryptographic algorithm by pseudo-random masks. That is, by using masks that are function f of the input to the algorithm. The implementation of f may be merged with the implementation of the cryptographic algorithm in order to prevent an attacker from influencing or inspecting the pseudo random mask. White-box implementations are well-suited for this.

Below white-box embodiments are described using the AES (Advanced Encryption Standard) block cipher, because AES has become a widely used standard for block ciphers. AES is a block cipher with a block size of 128 bits or 16 bytes. The plaintext is divided in blocks of 16 bytes which form the initial state of the encryption algorithm, and the final state of the encryption algorithm is the cipher text. At any given point in the encryption algorithm these 16 bytes are the state of the encryption algorithm. To conceptually explain AES, the bytes of the state are organized as a matrix of 4×4 bytes. AES includes a number of rounds, which depend on the key size. Each round includes similar processing steps operating on bytes, rows, or columns of the state matrix, each round using a different round key in these processing steps. In the discussion using AES as an example, it is noted that AES defines a round in a specific manner. In the embodiments below, a round is any grouping of steps that includes at least one non-linear mapping function, such as an S-box in AES. Accordingly, a round as described below includes one non-linear mapping function and any combination of other steps of the cryptographic function. Further, the boundary of the round may start with the non-linear mapping function, for example an S-box, or any other operation that may be merged with the non-linear mapping function, for example a key addition.

FIG. 1 illustrates some main processing steps of a round of AES. The processing steps include:

AddRoundKey 110—each byte of the state is XORed with a byte of the round key;

SubBytes 120—a byte-to-byte permutation using a lookup table;

ShiftRows 140—each row of the state is rotated a fixed number of bytes; and

MixColumns 150—each column is processed using a modulo multiplication in GF(28).

The steps SubBytes 120, ShiftRows 130, and MixColumns 150 are independent of the particular key used. The key is applied in the step AddRoundKey 110. Except for the step ShiftRows 140, the processing steps can be performed on each column of the 4×4 state matrix without knowledge of the other columns. Therefore, they can be regarded as 32-bit operations as each column consists of four 8-bit values. Dashed line 150 indicates that the process is repeated until the required number of rounds has been performed.

Each of these steps or a combination of steps may be represented by a lookup table or by a network of lookup tables. If the AddRoundKey 110 step is implemented by XORing with the round key, then the key is visible to the attacker in the white-box attack context. The AddRoundKey 110 step can also be embedded in lookup tables, which makes it less obvious to find out the key. In fact, it is possible to replace a full round of AES by a network of lookup tables. For example, the SubBytes 120, ShiftRows 130, and MixColumns 150 steps may be implemented using table lookups. Below a possible white-box implementation of AES in sufficient detail is discussed to describe the embodiments of the invention below, but further detailed descriptions of such an implementation are found in Chow 1. Also, other variations in the lookup table implementation may be used which are within the scope of the invention.

Both the table-based white-box implementations and the finite state machine implementations have the property that all intermediate values in the implementation are encoded (as compared to a standard implementation). Examples of white-box implementations using finite state machines are disclosed in U.S. Patent Publication 2007/0014394 entitled “Data Processing Method” and a presentation at the Re-trust Sixth Quarterly Meeting entitled “Synchrosoft MCFACT™ Secure Data Processing Technology” by Wulf Harder and Atis Straujums dated Mar. 11, 2008, which each are hereby incorporated by reference for all purposes as if fully set forth herein. FIG. 2 illustrates a white-box AES implementation with fixed encodings on the input of the rounds, i.e., on the input of the S-boxes. As shown, each of the 16 input bytes are encoded by f_(i) and each of the output bytes are encoded by g_(i).

In order to describe embodiments of the invention, a basic description of a table-based white-box AES implementation will be described. For a more detailed description of a method for implementing a table-based white-box AES see Chow 1. Chow 1 illustrates a specific implementation that breaks up certain functions using tables of specified sizes. It is well understood that various other divisions of the tables may be made resulting in different functions for the look-up tables and different sizes. Further, while the embodiments of the invention described below use a table-based white-box implementation of AES, other ciphers and cryptographic functions may be implemented according to the embodiments described. Also, other types of white-box implementations may be used instead of the table-base implementation, for example, a finite-state implementation.

The description of the table-based white-box AES is split into two steps. In the first step, a round of AES is described as a network of lookup tables. In the second step, the tables are obfuscated by encoding their input and output.

Step 1: Implementing AES as a Network of Lookup Tables.

AES operates on data blocks of 16 bytes. These are typically described as a 4×4 byte matrix, called the state including bytes x_(1,1), x_(1,2), x_(1,3), . . . x_(4,4). A round of AES as described above with respect to FIG. 1 include the following operations: AddRoundKey 110, SubBytes 120, ShiftRows 130, and MixColumns 140. The first two operations, AddRoundKey and SubBytes can be merged into a single T-box operation. That is, we can define a byte-to-byte function T_(i,j) for input byte x_(i,j) as T_(i,j)(x_(i,j))=S(x_(i,j)⊕k_(i,j)) where k_(i,j) is a single byte of a 16 byte round key based upon the AES key. Let y_(i,j) be the output of T_(i,j). The ShiftRows operations is just an index-renumbering of the output bytes y_(i,j). For ease of presentation, this operation is omitted in this description, but may be incorporated into the look-up table implementing T_(i,j) or implemented as a separate manipulation of the state matrix. In the MixColumns step, an output byte z_(i,j) of the round is computed from the 4 output bytes y_(1,j), y_(2,j), y_(3,j), and y_(4,j) via the algebraic expression z_(l,j)=MC_(l,1)·y_(1,j) ⊕MC_(l,2)·y_(2,j) ⊕MC_(l,3)·y_(3,j) ⊕MC_(l,4)·y_(4,j) in GF(28) for some constants MC_(l,r).

Now define a lookup table for each byte-to-byte function Q_(i,j,l)(x_(i,j))=MC_(l,i)·T_(i,j)(x_(i,j)) with i,j,l=1, 2, . . . , 16. Then any output byte z_(l,j) may be computed by XORing the results of these lookup tables, i.e., z_(l,j)=Q_(1,j,l)(x_(1,j))⊕Q_(2,j,l)(x_(2,j))⊕Q_(3,j,l)(x_(3,j)) ⊕Q_(4,j,l)(x_(4,j)). Note that the index i, j, l of Q-box can be interpreted as “the contribution of input byte i, j of a round to output byte 1, j of the round”. The XOR may be implemented to operate on each of two nibbles (i.e., 4-bit values) as a lookup table to reduce the size of the XOR tables. Accordingly, the Q-box may be implemented to produce output nibbles so that the size of the tables is reduced. Therefore, the computation of each output byte z_(l,j) of an AES-round has been described as a network of lookup tables. The network of lookup tables to compute a single output nibble of byte z_(2,3) is shown in FIG. 3.

FIG. 3 illustrates the computation of one output nibble by means of a network of look-up tables. The superscript index (1) in the Q-boxes indicates that the tables only provide the first nibble of the output of the Q-box. A set of input bytes x_(1,3), x_(2,3), x_(3,3) and x_(4,3) in the input state 310 are input into the Q-boxes 320, 322, 324, 326. The outputs u₁, u₂ of lookup tables 320 and 322 are fed into the XOR 330, and the outputs u₃, u₅ of lookup table 324 and XOR 330 are fed into the XOR 332. The outputs u₄, u₆ of table 326 and XOR 332 are fed into XOR 334. The output of XOR 334 is the first nibble of the output z_(2,3) of output state 340. The second nibble of the output z_(2,3) of output state 340 may be calculated in the same way using additional Q-boxes along with a similar XOR network. Further, additional sets of tables may be implemented to completely convert the input state 310 into the output state 340 by receiving a column of bytes from the input state and converting them into the output of the corresponding column of the output state.

Step 2: Obfuscating the Tables and the Intermediate Values

In the implementation depicted in FIG. 3, the key may easily be extracted from the Q-boxes. Just applying the inverse MixColumns multiplication and the inverse S-box to the output reveals the plain AddRoundKey operation. To prevent this, the input and outputs of all lookup tables are encoded with arbitrary bijective functions. This is described in Chow 1. This means that a lookup table is merged with an encoding function that encodes the output and with a decoding function that decodes the input. The encodings are chosen such that the output encoding of one table matches the input encoding assumed in the next tables. A portion of the implementation of FIG. 3 is depicted in FIG. 4 for the first round. In this example, the input to the round is not encoded in order to be compliant with AES, but the output of the round is encoded. The output encoding is handled in the next round. That is, unlike the first round, the second round (and the later rounds) assumes that the input is encoded. Alternatively, the first round may receive an encoded input. This input encoding must then be applied elsewhere in the software program containing the white-box implementation. Similarly, the last round may or may not include an output encoding depending on whether the output is to be AES compliant. Note that in the white-box implementation obtained, both the lookup tables and the intermediate values are obfuscated.

FIG. 4 illustrates a portion of the network of tables of FIG. 3 obfuscated by encoding the inputs and outputs. The lookup tables 420, 422, 424, 426 correspond to lookup tables 320, 322, 324, 326 of FIG. 3. The inputs of lookup tables 420, 422, 424, 426 are encoded by functions E₉, E₁₀, E₁₁, E₁₂, respectively. The outputs of lookup tables 420, 422, 424, 426 are encoded by functions f₁, f₂, f₃, f₄, respectively. XOR 430 corresponds to XOR 330. The inputs of XOR 430 decode input using f₁ ⁻¹ and f₂ ⁻¹. The output of XOR 430 is then encoded by function f₅. In a similar manner XORs 432, 434 have input decodings and output encodings as illustrated. The output z_(2,3) is encoded using f₇.

Standard white-box implementations, like the table-based ones described above, Chow et al., and the finite-state-machine-based ones described above, are vulnerable to the CPA attack as mentioned above. It is known in the literature that an intermediate value v may be protected against CPA attacks by masking v via v_(m)=v⊕m where m is a true random number. The embodiments described herein replace the true random mask by a mask that is a function of the input of the cryptographic algorithm. So, both the mask and the value that should be hidden are given by a function of the input of the algorithm. Accordingly, for each input the mask will be different and hence appear to an attacker to be random. This thwarts the CPA attack.

This may be more precisely formulated as follows. Let v be an intermediate value that is to be masked. Because v depends on the input x of the cryptographic algorithm, v may be written as v=g(x) for some function g. Next, v is masked via v_(m)=v⊕m, where mask m is specified by the input x of the cryptographic algorithm. That is, m=ƒ(x) for some function ƒ. For readability and as an example, the XOR operation ⊕ is used as the masking function, however, this operation may also be other binary operations.

When using a true random number generator, the literature states that the property that v_(m) is pairwise independent of V and m is needed. Informally, this means that v_(m) does not leak any information on v or m. If a good random number generator is used, the first property will be easy to satisfy. However, in the embodiments described herein, v and m are a function of the same parameter, viz.x. Hence, care must be taken. To formalize the property, the following notation is introduced. The value p_(a|b) is the probability that the masked value v_(m) equals a given that the intermediate value v equals b, and it may be calculated as follows:

$p_{a{b}} = {\frac{\#_{x}\left( {v_{m} = {{a\bigwedge v} = b}} \right)}{\#_{x}\left( {v = b} \right)} = {\frac{\#_{x}\left( {{\left( {{{g(x)} \otimes {f(x)}} = a} \right)\bigwedge{g(x)}} = b} \right)}{\#_{x}\left( {{g(x)} = b} \right)}.}}$

Analogously, the value {circumflex over (p)}_(a|b) is the probability that the masked value v_(m) equals a given that the mask m equals b, and it may be calculated as follows:

${\hat{p}}_{a{b}} = {\frac{\#_{x}\left( {v_{m} = {{a\bigwedge m} = b}} \right)}{\#_{x}\left( {m = b} \right)} = {\frac{\#_{x}\left( {{\left( {{{g(x)} \otimes {f(x)}} = a} \right)\bigwedge{f(x)}} = b} \right)}{\#_{x}\left( {{f(x)} = b} \right)}.}}$

Finally, the value p_(a) denotes the probability that the masked value v_(m) equals a, and it may be calculated as follows:

$p_{a} = {\frac{\#_{x}\left( {v_{m} = a} \right)}{\#_{x}} = {\frac{\#_{x}\left( \left( {{{g(x)} \otimes {f(x)}} = a} \right) \right)}{\#_{x}}.}}$

Now, the properties may be formalized as:

-   -   for all b for which v=b can occur and for all a: p_(a|b)=p_(a);         and     -   for all b for which m=b can occur and for all a: {circumflex         over (p)}_(a|b)=p_(a).

Although in the ideal case these properties are true, these properties may be relaxed a bit for practical situations. What is desired is that there is no difference in v_(m) for different underlying values v for a large number of traces, e.g., 1 million traces. Thus the properties may be relaxed to the following:

-   -   for all b for which v=b can occur and for all a:         p_(a|b)=p_(a)+ε, where ε is a small positive or negative number;         and     -   for all b for which m=b can occur and for all a: {circumflex         over (p)}_(a|b)=p_(a)+ε, where ε is a small positive or negative         number.

Furthermore, it is a goal to hide the intermediate value, but not the mask. So, the first of these two properties has to be satisfied. The second not per se.

What is still open in the formalization is value of ε. The value 1/ε is related to the number of traces needed in a CPA attack to exploit the dependence between v_(m) and v (or m). So, take |ε|<10⁻⁶ to let the number of required traces be larger than 1 million.

For readability, embodiments of masking intermediate values will be illustrated by extending the implementation visualized in FIG. 3. Encodings may next be applied as described above with respect to FIG. 4.

Now it will be shown how to mask the intermediate values u₅ and u₆ in FIG. 3. FIG. 5 illustrates a network that computes a pseudo-random mask m. The pseudo-random mask m is applied to the intermediate values.

The masking value m may be computed by m=ƒ(x₁, x₂, . . . , x₁₆)=h₁(x₁)⊕h₂(x₂)⊕ . . . ⊕h₁₆(x₁₆), where each h_(i) may be a random bijective function and x₁, x₂, . . . , x₁₆ are the 16 input bytes. As shown in FIG. 5 input bytes x₁ and x₂ are input into the functions h₁ 520 and h₂ 530. The outputs of the functions h₁ 520 and h₂ 530 are XORed together by XOR 510. The combination of the functions h₁ 520 and h₂ 530 and the XOR 510 are shown as a single block and hence may, for example, be implemented as a single lookup table or as a plurality of lookup tables. The function h₃ 550 then operates on the input byte x₃ and the output is then XORed by the XOR 540 with the output from the XOR 510. This process continues until the final input byte x₁₆ is input to the final function h₁₆ 570 and the XOR 560 produces the final mask value m.

FIG. 6 illustrates how the mask may be applied to the intermediate values present in the network of tables in FIG. 3. The Q-boxes 620, 622, 624, 626 correspond to the Q-boxes 320, 322, 324, 326 in FIG. 3. Further, the XORs 630, 632, 634 correspond to the XORs 330, 332, 334 in FIG. 3. In order to mask the intermediate values u₁, u₅, and u₆ an additional XOR 640 may be added to the network of FIG. 3. The XOR 640 XORs the mask m and u₁ resulting in the value u₁⊕m, which masks the intermediate value u₁. Because the one input to XOR 630 is the masked value u₁⊕m the output from the XOR 630 is now the masked value u₅⊕m. The same is true for XOR 632 and XOR 634 which produce masked outputs. The output of 634 is z_(2,3)⊕m which may be input into the XOR 650 which removes the mask by XORing with the mask value m (z_(2,3) ⊕M) to result in the unmasked value z_(2,3), which is then input to the S-box of the next round of operation.

The value z_(2,3) is input to the S-box of the next round. Removing the mask before using z_(2,3) as input to the S-box results in a functionally correct implementation. To further improve security, it may be desirable to not remove the mask at the input of the S-box. This may be realized by using multiplicative masks instead of additive masks and by applying embodiments found in U.S. patent application Ser. Nos. 14/219,606 and 14/484,925, which are hereby incorporated by reference for all purposes as if fully set forth herein.

In other embodiments, the calculation of the mask may be varied for application to different intermediate values. For example, the functions h₁, h₂, . . . , h₁₆ may be randomly selected for each different calculation of a mask value for application to different intermediate values of the cryptographic function.

What remains, is showing that for u₅ and u₆ the added masks satisfy at least the first of the following two properties:

-   -   For all b for which v=b can occur and for all a we have         p_(a|b)=p_(a)+ε, where ε is a small positive or negative number;         and     -   For all b for which m=b can occur and for all a we have         {circumflex over (p)}_(a|b)=p_(a)+ε, where ε is a small positive         or negative number.

Consider the first condition for intermediate value u₅. From the definition of Q_(i,j,l) it follows that the tables Q_(1,3,2) ⁽¹⁾ and Q_(2,3,2) ⁽¹⁾ which deliver u₁ and u₂ with u₅=u₁ ⊕u₂ are both surjective. Hence, for each b if follows that v=b may occur.

For example, fix values x₁ and x₂. Then by varying x₃, x₄, . . . , x₁₆ each value for the mask m occurs with a probability of exactly

$\frac{1}{2^{4}} = {1/16}$

(this is because m is a 4 bit value). This means that irrespective of what is assumed regarding the values of x₁ and x₂, the probability that a mask has a given value is given by 1/16. As a result, the probability that m is some value C given that v=u₅=b for any value b is 1/16 as well. The following may now be derived:

p _(a|b) =P(v _(m) =a|u ₅ =b)=P(m=a⊕b|u ₅ =b)=1/16

for any a, b. By the law of total probability, it follows that if a conditional probability is constant for all possible conditions, then also the unconditional probability equals this constant: hence, p_(a)=1/16. This proves p_(a|b)=p_(a)+ε for ε=0.

Next, consider the second condition above (which, as mentioned before, is not essential for the invention). As indicated, the probability that m has some value C is 1/16 irrespective of the value of u₅. That is, the variables m and u₅ are independent. Thus the following may be computed:

{circumflex over (p)} _(a|b) =P(v _(m) a|m=b)=P(u ₅ =a⊕b|m=b)=P(u ₅ =a⊕b)

In the above described definition of Q_(i,j,l) each of the 16 possible nibble values occurs exactly the same number of times. Hence, P(u₅=c) equals the constant value 1/16. This yields {circumflex over (p)}_(a|b)=1/16 for any a, b. Using, as before the law of total probability, this gives {circumflex over (p)}_(a|b)={circumflex over (p)}_(a)+ε for ε=0.

This completes the proof that u₅ was masked according to the embodiments described herein. This proof also applies to the masking of the

A method according to the embodiments of the invention may be implemented on a computer as a computer implemented method. Executable code for a method according to the invention may be stored on a computer program medium. Examples of computer program media include memory devices, optical storage devices, integrated circuits, servers, online software, etc. Accordingly, a white-box system may include a computer implementing a white-box computer program. Such system, may also include other hardware elements including storage, network interface for transmission of data with external systems as well as among elements of the white-box system.

In an embodiment of the invention, the computer program may include computer program code adapted to perform all the steps of a method according to the invention when the computer program is run on a computer. Preferably, the computer program is embodied on a non-transitory computer readable medium.

Further, because white-box cryptography is often very complicated and/or obfuscated it is tedious for a human to write. It is therefore of advantage to have a method to create the cryptographic system according to the embodiments of the invention in an automated manner.

A method of creating the cryptographic system according to the invention may be implemented on a computer as a computer implemented method, or in dedicated hardware, or in a combination of both. Executable code for a method according to the invention may be stored on a computer program medium. In such a method, the computer program may include computer program code adapted to perform all the steps of the method when the computer program is run on a computer. The computer program is embodied on a non-transitory computer readable medium.

The cryptographic system described herein may be implemented on a user device such as a mobile phone, table, computer, set top box, smart TV, etc. A content provider, such as a television network, video stream service, financial institution, music streaming service, etc., may provide software to the user device for receiving encrypted content from the content provider. That software may have the encryption key embedded therein as described above, and may also include binding strings as described above. Then the content provider may send encrypted content to the user device, which may then decrypt using the supplied software and use the content.

FIG. 7 illustrates a system for providing a user device secure content and a software application that processes the secure content. The system includes a content server 700, application server 780, user devices 750, 752, and a data network 740. The user devices 750, 752 may request access to secure content provided by the content server 700 via data network 740. The data network can be any data network providing connectivity between the user devices 750, 752 and the content server 700 and application server 780. The user devices 750, 752 may be one of a plurality of devices, for example, set top boxes, media streamers, digital video recorders, tablets, mobile phones, laptop computers, portable media devices, smart watches, desktop computers, media servers, etc.

The user request for access may first require the downloading of a software application that may be used to process the secure content provided by the content server 700. The software application may be downloaded from the application server 780. The software application may be obscured using the techniques described above as well as operate as described above. Once the user devices 750, 752 install the software application, the user device may then download secure content from the content server 700 and access the secure content using the downloaded software application. For example, the downloaded software application may perform decryption of encrypted content received from the content server. In other embodiments, the software application may perform other secure operations, such as for example, encryption, digital signature generation and verification, etc.

The content server 700 may control the access to the secure content provided to the user devices 750, 752. As a result when the content server 700 receives a request for secure content, the content server 700 may transmit the secure content to the requesting user device. Likewise, the application server 720 may control access to the software application provided to the user devices 750, 752. As a result when the content server 720 receives a request for the software application, the application server 720 may transmit the software application to the requesting user device. A user device requesting the software application or secure content may also be authenticated by the respective servers, before providing the software application or secure content to the user device.

The content server 700 may include a processor 702, memory 704, user interface 706, network interface 710, and content storage 712 interconnected via one or more system buses 780. It will be understood that FIG. 7 constitutes, in some respects, an abstraction and that the actual organization of the components of the device 700 may be more complex than illustrated.

The processor 702 may be any hardware device capable of executing instructions stored in memory 704 or storage 712. As such, the processor may include a microprocessor, field programmable gate array (FPGA), application-specific integrated circuit (ASIC), or other similar devices.

The memory 704 may include various memories such as, for example L1, L2, or L3 cache or system memory. As such, the memory 702 may include static random access memory (SRAM), dynamic RAM (DRAM), flash memory, read only memory (ROM), or other similar memory devices.

The user interface 706 may include one or more devices for enabling communication with a user such as an administrator. For example, the user interface 706 may include a display, a mouse, and a keyboard for receiving user commands.

The network interface 710 may include one or more devices for enabling communication with other hardware devices. For example, the network interface 710 may include a network interface card (NIC) configured to communicate according to the Ethernet protocol. Additionally, the network interface 710 may implement a TCP/IP stack for communication according to the TCP/IP protocols. Various alternative or additional hardware or configurations for the network interface 710 will be apparent.

The content storage 712 may include one or more machine-readable content storage media such as read-only memory (ROM), random-access memory (RAM), magnetic disk storage media, optical storage media, flash-memory devices, or similar storage media. In various embodiments, the content storage 712 may store content to be provided to users.

The application server 720 includes elements like those in the content server 700 and the description of the like elements in the content server 700 apply to the application server 720. Also, the content storage 712 is replaced by application storage 732. Further, it is noted that the content server and applications server may be implemented on a single server. Also, such servers may be implemented on distributed computer systems as well as on cloud computer systems.

Any combination of specific software running on a processor to implement the embodiments of the invention, constitute a specific dedicated machine.

As used herein, the term “non-transitory machine-readable storage medium” will be understood to exclude a transitory propagation signal but to include all forms of volatile and non-volatile memory. Further, as used herein, the term “processor” will be understood to encompass a variety of devices such as microprocessors, field-programmable gate arrays (FPGAs), application-specific integrated circuits (ASICs), and other similar processing devices. When software is implemented on the processor, the combination becomes a single specific machine.

It should be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the invention.

Although the various exemplary embodiments have been described in detail with particular reference to certain exemplary aspects thereof, it should be understood that the invention is capable of other embodiments and its details are capable of modifications in various obvious respects. As is readily apparent to those skilled in the art, variations and modifications can be effected while remaining within the spirit and scope of the invention. Accordingly, the foregoing disclosure, description, and figures are for illustrative purposes only and do not in any way limit the invention, which is defined only by the claims. 

What is claimed is:
 1. A non-transitory machine-readable storage medium encoded with instructions for execution by a keyed cryptographic operation by a cryptographic system mapping an input message to an output message, comprising: instructions for receiving input data for the keyed cryptographic operation; instructions for calculating a first mask value based upon the input data; and instructions for applying the first mask value to a first intermediate value of the keyed cryptographic operation.
 2. The non-transitory machine-readable storage medium of claim 1, further comprising setting a threshold value ε, wherein the mask value based upon the input is calculated such that for all values of b: |p _(a|b) −p _(a)|≦ε, where p_(a|b) is the number of input values for which the masked value equals a given that the first intermediate value equals b divided by the number of values for which the intermediate value equals b, and where p_(a) is the number of input values for which the masked value equals a divided by the number of input values.
 3. The non-transitory machine-readable storage medium of claim 2, wherein the threshold value ε is selected based upon an expected number of traces collected by an attacker.
 4. The non-transitory machine-readable storage medium of claim 1, wherein the cryptographic function is the Advanced Encryption Standard (AES).
 5. The non-transitory machine-readable storage medium of claim 1, wherein the cryptographic function is the Data Encryption Standard (DES).
 6. The non-transitory machine-readable storage medium of claim 1, wherein the input data and has N portions and wherein instructions for calculating a first mask value based upon the input data further comprises: instructions for applying N bijective functions to each of the N portions of the input data; and instructions for combining outputs of the N bijective functions resulting in the first mask value.
 7. The non-transitory machine-readable storage medium of claim 1, further comprising: instructions for calculating second mask value based upon the input data that is different from the first mask value; and instructions for applying the second mask value to a second intermediate value of the keyed cryptographic operation.
 8. The non-transitory machine-readable storage medium of claim 1, wherein the keyed cryptographic operation includes a plurality of rounds with each round including a plurality of substitution functions, the outputs of the substitution functions are combined to produce an output of the round, and the outputs of the substitution functions are the first intermediate value.
 9. The non-transitory machine-readable storage medium of claim 8, further comprising: applying the first mask value to the output of the round to remove the first mask value.
 10. The non-transitory machine-readable storage medium of claim 8, wherein the keyed cryptographic operation further comprises: a first round of the keyed cryptographic operation producing a first masked output; and a second round receiving the first masked output wherein the second round compensates for the masking of the first masked output.
 11. A method of controlling a server that provides an application that implements a keyed cryptographic operation by mapping an input message to an output message, comprising: receiving a request from a user for the application that implements a keyed cryptographic operation by mapping an input message to an output message; and providing the user the application that implements a keyed cryptographic operation by mapping an input message to an output message, wherein the application was created by: receiving input data for the keyed cryptographic operation; calculating a first mask value based upon the input data; and applying the first mask value to a first intermediate value of the keyed cryptographic operation.
 12. The method of claim 11, wherein the application was further created by setting a threshold value ε, wherein the mask value based upon the input is calculated such that for all values of b: |p _(a|b) −p _(a)|≦ε, where p_(a|b) is the number of input values for which the masked value equals a given that the first intermediate value equals b divided by the number of values for which the intermediate value equals b, and where p_(a) is the number of input values for which the masked value equals a divided by the number of input values.
 13. The method of claim 12, wherein the threshold value E is selected based upon an expected number of traces collected by an attacker.
 14. The method of claim 11, wherein the input data and has N portions and wherein the application was further created by: applying N bijective functions to each of the N portions of the input data; and combining outputs of the N bijective functions resulting in the first mask value.
 15. The method of claim 11, wherein the application was further created by: calculating second mask value based upon the input data that is different from the first mask value; and applying the second mask value to a second intermediate value of the keyed cryptographic operation.
 16. The method of claim 11, wherein the keyed cryptographic operation includes a plurality of rounds with each round including a plurality of substitution functions, the outputs of the substitution functions are combined to produce an output of the round, and the outputs of the substitution functions are the first intermediate value.
 17. The method of claim 16, wherein the application was further created by: applying the first mask value to the output of the round to remove the first mask value.
 18. The method of claim 16, wherein the keyed cryptographic operation further comprises: a first round of the keyed cryptographic operation producing a first masked output; and a second round receiving the first masked output wherein the second round compensates for the masking of the first masked output.
 19. A method for a keyed cryptographic operation by a cryptographic system mapping an input message to an output message, comprising: receiving input data for the keyed cryptographic operation; calculating a first mask value based upon the input data; and applying the first mask value to a first intermediate value of the keyed cryptographic operation.
 20. The method claim 19, further comprising setting a threshold value ε, wherein the mask value based upon the input is calculated such that for all values of b: |p _(a|b) −p _(a)|ε, where p_(a|b) is the number of input values for which the masked value equals a given that the first intermediate value equals b divided by the number of values for which the intermediate value equals b, and where p_(a) is the number of input values for which the masked value equals a divided by the number of input values.
 21. The method of claim 20, wherein the threshold value E is selected based upon an expected number of traces collected by an attacker. 